The invention relates to a built-up camshaft having cam discs which are secured to a preferably tubular shaft body under mechanical prestress, are of approximately annular design and are hardened before they are secured to the shaft body. The invention further relates to a method of inductively hardening the boundary layer of the outer periphery of cam discs for a built-up camshaft, which cam discs are of approximately annular design and can be secured after the hardening to a preferably tubular shaft body under mechanical prestress. In the boundary zone to be hardened, the cam disc is heated to transformation temperature by rotation in an alternating magnetic field. The magnetic field is concentric to the rotation axis and is produced by a ring inductor which can be loaded by alternating current. The cam disc is heated on account of corresponding alternating eddy currents induced in the cam disc and is then quenched by an external quenching medium, for hardening the boundary layer of the cam disc for a built-up camshaft.
In the course of further developing internal combustion engines having less consumption and lower emissions, the engine developers are adopting increasingly "sharper", i.e. steeper rising, cams and cam followers having rollers, both of which lead to greater loading of the cams, in particular in the flank region. The higher cam loading requires better materials at least for the cams. A high-grade cam material on the one hand and normal engineering steel for the shaft body on the other hand lead to a composite construction of the camshaft, the so-called "built-up" camshaft. Apart from the price advantage over a forged or cast camshaft, which is made entirely of the high-grade material, a built-up camshaft also offers weight advantages.
German Patent document DE 37 17 190 C2 discloses a built-up camshaft in which sintered cam discs are pressed axially onto a tubular shaft body and are additionally secured in a positive-locking manner in the direction of rotation. This positive locking is brought about automatically by the pressing-on. Specifically, to secure a cam disc to the shaft tube, peripheral grooving is rolled in its periphery at the relevant axial position and over a length corresponding to the width of the cam disc. The peripheral grooving enlarges the effective outside diameter of the shaft tube locally in a deliberate manner. The cam disc is then pressed axially in the desired peripheral position onto this cam seat enlarged in diameter. Since the cam discs are designed as sintered bodies, they contain, on account of the manufacturing process, many small pores and material interruptions which adversely affect the service life of the cams during alternating loading due to a high Hertzian stress. Apart from that, formed sintered bodies, due to the process, are expensive when compared with workpieces made of solid material. Production of the formed bodies by the sintering method is only worthwhile when comparatively complicated forms have to be produced. This, however, does not apply in the case of cam discs.
Therefore steel cam discs are adopted, which for loading and wear reasons are inductively hardened at the boundary layer over their entire outer periphery. In order for the perforated cam discs to adhere to the tube of the shaft body completely free of play, even after a prolonged period of use, the cam discs should not be fully hardened down to the inner surfaces of the aperture mounted on the tube. On the contrary, in addition to the ductile shaft body, the inner surface of the cam disc should also maintain a certain plasticity. However, this hardening of the boundary layer of the cam disco must not be effected after the mounting of the cam discs on the shaft body, since otherwise the firmly adhering bond would be loosened in an inadmissible manner by the heat treatment.
Due to the pressing-on or other frictional, possibly positive-locking, securing of the cam discs hardened at the boundary layer on the outer periphery, tensile stresses occur in the annular cam disc. In conventionally induction-hardened cam discs, through the addition of the operational load likewise causing tensile stresses, the tensile stresses occurring in the annular cam disc lead on the whole to tensile stresses which lie close to the fatigue strength of the material and, accordingly, lead very quickly to surface cracks or, in the extreme case, to the cam-disc ring being torn apart. Therefore, built-up camshafts of the generic type having cam discs inductively hardened at the boundary layer on the outer periphery have been unable to gain acceptance hitherto in series application for internal combustion engines, at least in the lightweight construction required for passenger cars and at least in conditions of use with high surface pressures, e.g. roller tappets.
German Patent document DE 37 17 534 C2 likewise shows a built-up camshaft in which the shaft tube is expanded by hydraulic internal pressure applied locally in a deliberate manner. The shaft tube is pressed on the inside under prestress against the inside of the cam disc. Here, the cam disc, if possible, is only to be deformed elastically. After hydraulic relief of the shaft tube, the cam disc radially springs back and adheres firmly to the plastically expanded shaft tube. The cam disc is already largely machine-finished before being secured to the shaft tube. If need be, grinding can still be carried out after the joining. In particular, the cam disc is already hardened at the outer periphery before the joining. This German Patent document mentions with reference to an earlier publication that the hardened surface of the cams tends to form cracks during the elastic deformation. To avert this risk of crack formation, it is therefore proposed to axially spread apart the seating region of the cam on the shaft tube on the one hand, and the actual cam body on the other hand, and to configure the seating region to be radially yielding and the actual cam region to be radially rigid. However, this configuration of a built-up camshaft fails when--as in most cases--the cams have to be accommodated on the shaft tube at a very small axial distance apart.
There is therefore needed an improved camshaft to the effect that a useful service life of the camshaft or the cam disc can be expected. Furthermore, there is needed an improved method for inductively hardening the boundary layer along the same lines.
These needs are met according to the present invention by a built-up camshaft having cam discs which are secured to a preferably tubular shaft body under mechanical prestress, are of approximately annular design and are hardened before they are secured to the shaft body. The cam discs are inductively hardened at a boundary layer on their outer periphery in such a manner that the residual compressive stresses, induced by the hardening, in the hardened boundary zone near the surface are so high that, after the cam discs are mounted on the shaft body, the superimposing residual tensile stresses, induced by the joining, in the boundary zone only partly compensate the original residual compressive stresses, i.e. that there are still only residual compressive stresses in the boundary zone near the surface even after the mounting.
The method according to the present invention for inductively hardening the boundary layer of the outer periphery of the cam discs for a built-up camshaft effects heating in two stages in order to produce sufficiently high residual compressive stress in the boundary zone of the cam disc. In a first stage, the entire annular body of the cam disc is preheated to at least 250.degree. C. but at most to a limiting temperature lying below the transformation temperature by loading the cam disc with a medium-frequency alternating magnetic field. In a subsequent second stage, only the radially outer boundary zone of the preheated cam disc is heated up to a temperature lying at or above the transformation temperature by loading the cam disc with a second alternating magnetic field changed in its intensity and/or its frequency relative to the alternating magnetic field of the first stage.
The advantages of the improved camshaft according to the invention lie in the fact that the corresponding camshafts have a justifiable service life only with permanent residual compressive stresses in the hardened boundary zone of the mounted cam discs. The high residual compressive stresses establishing the service life can be produced during the hardening by the hardening method. The surprising solution to the strength problem lies in forcing residual compressive stresses near the boundary even after the pressing-on. These can be achieved by the induction hardening being run, before the pressing-on, to extremely high residual compressive stress at small hardness penetration depth. The tensile stresses induced by the joining must therefore be overcompensated as it were by "allowance" being made for an appropriately high residual compressive stress during the hardening. The especially high residual compressive stresses during the hardening of a thin boundary layer are achieved by the two-stage workpiece heating during the induction hardening with full cross-section preheating of the cam ring and the actual induction hardening, building up thereon, of the boundary layer.
The magnitude of the residual compressive stresses can be determined by non-destructive radiographic testing in a thin boundary layer down to a depth of about 5 .mu.m. Although this non-destructive radiographic testing method and its handling are not exactly simple, it has been generally known for a long time. In the relevant literature, the following publications can be referred to:
1) E. Macherauch, P. Mueller: Das sin.sup.2 .psi.-Verfahren der roentgenographischen Spannungsmessung [The sine.sup.2 .psi.-method of radiographic stress measurement], Zeitschrift fuer angewandte Physik 13 (1961), pages 305 to 312;
2) Book: Eigenspannunge(n and Lastspannungen [Residual stresses and load stresses], Editor: v. Hauk, E. Macherauch, Carl Hanser Verlag, Munich, Vienna (1982);
3) B. Scholtes: Roentgenographische Spannungsermittlung, ihre Grundlagen and Anwendungen [Radiographic stress determination, its fundamentals and applications], in the book: Roentgen--und Elektronenbeugung [X-ray and electron diffraction], 65-85, editor S. Steeb, series Rontakt und Studium, volume 144, expert Verlag, Sindelfingen (19xx);
4) A Useful Guide for X-Ray Stress Evaluation. In: Advances in X-Ray Analysis 27 (1984), pages 81 to 99, Plenum Publishing Corp; and
5) Residual Stresses in Science and Technology, editor: E. Macherauch, V. Hauk, DGM-Informationsgesellschaft, Oberursel (1987).
If the depth profile of the residual stresses is also to be determined, the surface must be removed in layers at least locally at the measuring point in a mechanically non-reactive manner, e.g. by an electrochemical etching operation. The residual stress prevailing at the exposed surface must be determined radiographically in between for each layer. The depth profile of the residual stresses can thus only be determined in a destructive manner.